Today we will take a look a little bit into history and compare fast charging of three iterations of the BMW i3 (21.6, 33.2 and 42.2 kWh battery versions) and the related MINI Cooper SE model (32.6 kWh).
Those small electric cars are not expected to fast charge very quickly, but we would like to check whether BMW upgraded them in any way and what they offer. There are a few interesting findings in this report - small bricks in the foundation for further reports on newer models.
Data for this analysis comes from Fastned's fast-charging network, which is also an invaluable source for info about charging.
Let's first see the latest BMW i3 model, and then compare all four cars together.
Charging power vs state-of-charge (SOC)
The 2019 BMW i3 (44.2 kWh battery version) has a very flat charging curve with a peak charging power of 50 kW.
State-of-charge (SOC) vs time
Charging from 20% to 80% SOC takes about half an hour.
Average charging power vs state-of-charge (SOC)
The average power in the very important range from 20% to 80% SOC is 47 kW, which is 94% of the peak value.
The flat charging curve and high average to peak power ratio indicate that the battery probably could charge much faster if only the charging system/all the onboard cables would allow for it.
C-rate vs state-of-charge (SOC)
The peak C-rate* - charging power in relation to the total battery capacity of 42.2 kWh - is about 1.2C.
The average C-rate when charging from 20% to 80% SOC is 1.1C.
*C-rate tells us how the charging power relates to the battery pack capacity. For example: 1C is 1-hour charging power (current), when the power value in kW is equal to the battery pack capacity in kWh. 2C would be enough to recharge in half an hour.
The net battery capacity of 37.9 kWh stands for about 90% of the total battery capacity.
Range replenishing speed vs state-of-charge (SOC)
The rate of replenishing range depends on the energy consumption and the energy consumption depends on the use case.
In the case of i3, we can compare the values for WLTP and EPA ratings:
- WLTP
Taking into consideration the WLTP range of 310 km (193 miles) and available battery capacity of 37.9 kWh, we can assume energy consumption of 122 Wh/km (197 Wh/mile).
The effective average speed of range replenishing when charging from 20% to 80% SOC would be 6.4 km/minute (4 miles/minute). - EPA Combined
Taking into consideration the EPA Combined range of 153 miles (246 km) and available battery capacity of 37.9 kWh, we can assume energy consumption of 248 Wh/mile (154 Wh/km).
The effective average speed of range replenishing when charging from 20% to 80% SOC would be 3.1 miles/minute (5 km/minute). - EPA Highway
Taking into consideration the EPA Highway range of 136.4 miles (219 km) and available battery capacity of 37.9 kWh, we can assume energy consumption of 278 Wh/mile (173 Wh/km).
The effective average speed of range replenishing when charging from 20% to 80% SOC would be 2.8 miles/minute (4.5 km/minute).
Comparisons with other EVs
Comparison of charging power
The first comparison - of charging power curve - of three iterations of the BMW i3 and the MINI Cooper SE, reveals that BMW has increased the battery capacity (through using more energy dense cells), but probably did not touch the charging systems.
As a result, the power looks like it would be capped at 50 kW. We guess that the reason for that is to minimize costs on one hand and a conclusion that 50 kW for this type of cars is enough.
However, the charging curve has improved solely through the switch to a bigger battery capacity, because, as we can see, the bigger the capacity, the longer (to a higher SOC) the car was able to accept the peak value. The 22 kWh was falling at over 60% SOC, 33 kWh at around 80% SOC, while the 42 kWh was able to go up to almost 90% SOC.
Comparison of C-rate
If the charging power is similar, then clearly the packs with higher capacity will be operating in "easy" mode, at under 1.5C, compared to the original 22 kWh battery (over 2C).
A quick summary proves that the average charging power from 20% to 80% SOC improved with the increase of battery pack capacity. In the case of 2019 i3, it's 20% higher than in 2013 i3.
| DC Fast Charging Comparison by InsideEVs | |||||
| Model [data source] | Drive / Battery (kWh) | Max Power | Avg Power (20-80%) | Max C-Rate | Avg C-Rate (20-80%) |
| 2013 BMW i3 (22 kWh) [Fastned] | RWD 21.6 kWh | 47 kW | 39 kW | 2.2 | 1.8 |
| 2017 BMW i3 (33 kWh) [Fastned] | RWD 33.2 kWh | 50 kW | 46 kW | 1.5 | 1.4 |
| 2019 BMW i3 (42 kWh) [Fastned] | RWD 42.2 kWh | 50 kW | 47 kW | 1.2 | 1.1 |
| 2020 MINI Cooper SE [Fastned] | FWD 32.6 kWh | 49 kW | 45 kW | 1.5 | 1.4 |
Comparison of range replenishing speed
The speed of replenishing range is not particularly high, but high enough for the purpose of city cars that probably fast charge rarely.
A quick summary also for various test cycles:
| DC Fast Charging Comparison by InsideEVs | |||||
| Model [data source] | Drive / Battery (kWh) | Avg Power (20-80%) | WLTP range rep. rate (20-80%) | EPA range rep. rate (20-80%) | EPA Hgw range rep. rate (20-80%) |
| 2013 BMW i3 (22 kWh) [Fastned] | RWD 21.6 kWh | 39 kW | N/A | 4.5 km/min (2.8 mi/min) | 4.0 km/min (2.5 mi/min) |
| 2017 BMW i3 (33 kWh) [Fastned] | RWD 33.2 kWh | 46 kW | 7.2 km/min 4.5 mi/min | 5.2 km/min (3.2 mi/min) | 4.6 km/min (2.9 mi/min) |
| 2019 BMW i3 (42 kWh) [Fastned] | RWD 42.2 kWh | 47 kW | 6.4 km/min 4.0 mi/min | 5.0 km/min (3.1 mi/min) | 4.5 km/min (2.8 mi/min) |
| 2020 MINI Cooper SE [Fastned] | FWD 32.6 kWh | 45 kW | 6.1 km/min 3.8 mi/min | 4.6 km/min (2.9 mi/min) | 4.3 km/min (2.7 mi/min) |
Conclusions
The conclusion from today's analysis is that the new BMW i3/MINI charges faster thanks to higher battery capacity, despite the charging system being capped at 50 kW.
Here is all the data for the currently available BMW i3 and MINI Cooper SE:
| 2019 BMW i3 :: DC Fast Charging Summary by InsideEVs Drive: RWD; Battery pack (net / total): 37.9 / 42.2 kWh [Data source: Fastned] | |
| Peak Power Peak C-rate Average Power (20-80% SOC) Average-to-Peak Power Average C-rate (20-80% SOC) | 50 kW 1.2 47 kW 94% 1.1 |
| Range Replenishing Speed (Average 20-80% SOC): | |
| WLTP EPA Combined EPA Highway | 6.4 km/min (4.0 mi/min) 5.0 km/min (3.1 mi/min) 4.5 km/min (2.8 mi/min) |
| 2020 MINI Cooper SE :: DC Fast Charging Summary by InsideEVs Drive: FWD; Battery pack (net / total): 28.9 / 32.6 kWh [Data source: Fastned] | |
| Peak Power Peak C-rate Average Power (20-80% SOC) Average-to-Peak Power Average C-rate (20-80% SOC) | 49 kW 1.5 45 kW 92% 1.4 |
| Range Replenishing Speed (Average 20-80% SOC): | |
| WLTP EPA Combined EPA Highway InsideEVs 70 mph | 6.1 km/min (3.8 mi/min) 4.6 km/min (2.9 mi/min) 4.3 km/min (2.7 mi/min) 4.6 km/min (2.8 mi/min) |
See also
General info:
* Some values on the charts are estimated from the data source.
** Temperature of the battery cells might highly negatively affect charging capabilities. We don't have data about temperatures of the battery at the beginning and during the charging process. In cold or hot weather, as well as after driving very dynamically, charging power might be significantly lower than shown on the charts (in extreme cases charging might be impossible until the battery temperature will not return to an acceptable level).